1. Field of the Invention
The instant invention generally relates to a thin-film device and a method of fabricating the thin-film device. More specifically, the instant invention relates to a method of fabricating a thin-film device having a film layer that has bulk material properties.
2. Description of the Related Art
There are various possible applications of “smart” materials, such as piezoelectric materials, both in micro-electro-mechanical systems (MEMS) devices and in solid state devices. From a MEMS device perspective, piezoelectricity provides several advantages over electromagnetic or electrostatic conversion phenomenon, since its electromechanical conversion phenomena is fairly simple and effective. Just to name a few of these advantages; the stroke of an actuator is not limited by a pulling voltage as in electrostatic actuators, the resultant force is considerably higher, there is no need for a bulk heavy electromagnet in the design, and it is much more practical to scale down devices. From a solid-state device perspective, a ferroelectric material layer (which is a type of piezoelectric material) can modulate an adjacent semiconductor layer through induced charge, and opens possibilities of controlled band bending, memory device applications, and various sensor applications.
Typical fabrication methods for piezoelectric thin films involve great challenges such as a need for high temperature processing, propensity of stress-induced cracking in the piezoelectric thin films, limited film thickness, and reliability and repeatability issues. The piezoelectric thin films produced through the typical fabrication methods also suffer from low piezoelectric coefficients. Several methods are known for fabrication of piezoelectric thin films on silicon substrates. A few of the most widely-known fabrication methods are sol-gel deposition, direct writing (ink-jet printing), screen printing, sputtering, electrophoretic deposition, and epitaxial growth. Unfortunately, as listed in Table 1, all of these fabrication methods result in considerably lower piezoelectric properties in the piezoelectric thin films than the same material's bulk properties. Another problem is that among these fabrication methods, except screen printing and electrophoretic deposition, thickness of the piezoelectric film layers is limited to about 1 μm to 3 μm. Additionally, high temperature annealing processes are common for the piezoelectric thin films fabricated through the above methods to form a preferred crystal orientation in the piezoelectric thin films and to initiate piezoelectric properties in the piezoelectric thin films. Such high temperature annealing processes constrain an ability to fabricate the piezoelectric thin films on substrates in a post-CMOS fabrication process, under which conditions the substrates include heat-sensitive components such as integrated circuits disposed thereon.
TABLE 1DepositionPiezoelectricMaterialMin ProcessMethodsCoupling k31ThicknessTemperatureProcess ChallengesBulk Ceramics0.15-0.35>100 μmBondingDifficult to Achieve a(Wide rangeTemperatureReliable low temp. bond,of selection)Limited to Thick LayersScreen Printing<0.09010-100 μm600° C.-900° C.Poor Pattern resolution,Molding/CastingRequires a SlurryComposition,Limited to High TemperatureBondingSol-gel PZT0.060-0.090<3 μm400° C.-700° C.High stress, Shrinkage,Cracking, Substrate Effect,Limited to High TemperatureBondingSputtered PZT0.070<2-3 μm450° C.-650° C.Crystal orientationUniformity,Limited to High TemperatureBondingSputtered AlN0.017-0.030<2 μm 20° C.-400° C.Limited to Non-ferroelectricMaterialsLimited to Small ThicknessSputtered ZnO0.049<2 μm 20° C.-275° C.Fast diffusion of Zn,Low DC Resistivity,Limited to Small Thickness
As compared to piezoelectric thin films deposited through the above-reference fabrication methods, bulk piezoelectric materials provide greater electromechanical force, structural strength, and charge capacity, which are highly desirable properties in many MEMS devices including high-force actuators, harsh-environmental sensors, and micro-power scavengers. Physical properties of other bulk “smart” materials, such as thermoelectric, electrostrictive, magnetoelastic, ferromagnetic, ferromagnetic, and shape memory materials, are similarly improved over their thin-film counterparts. In view of the superior physical properties of the bulk piezoelectric materials as compared to piezoelectric thin films fabricated through the above-referenced methods, it is desirable to integrate bulk piezoelectric materials into thin-film devices.
Thin bulk piezoelectric materials have been integrated on MEMS devices and solid state devices in the past, but existing methods have suffered from one or more of the following problems: excessive bonding temperatures for bonding the thin bulk piezoelectric materials directly on the devices, an inability to successfully bond the thin bulk piezoelectric materials to the devices without experiencing cracking or delamination, an inability to directly form the thin bulk piezoelectric materials on the devices, and associated manufacturing inefficiencies stemming from an inability to form the thin bulk piezoelectric materials directly on the devices.
In one specific example of a known method, a bulk piezoelectric material is deposited and bonded onto a substrate. After bonding the bulk piezoelectric material to the substrate, the bulk piezoelectric material is thinned to obtain a thin piezoelectric material. An electrode is deposited on top of the thin piezoelectric material to form a thin piezoelectric device, which is subsequently bonded to a MEMS device. The thin piezoelectric device is formed outside of the CMOS environment (i.e., the substrate upon which the thin piezoelectric device is formed is not a MEMS device) and employs high temperature sintering processes to initially bond the bulk piezoelectric material to the substrate. However, the MEMS devices have heat-sensitive components disposed thereon which cannot withstand the temperatures required by the sintering process such that it would not be possible to form the thin piezoelectric devices directly on the MEMS devices in this method. Furthermore, the thin piezoelectric device is completely formed at the stage in which the device is bonded to the MEMS device such that no modification of the piezoelectric material is possible once the thin piezoelectric device is bonded to the MEMS device.
In view of the foregoing, there remains an opportunity to further improve upon existing methods of fabricating thin-film devices that include bulk piezoelectric materials.